A phase locked loop (PLL) is typically an automatic control system capable of tracking the frequency and phase of an input signal, and is widely utilized in the field of frequency control. It traces and locks the phase and frequency of an output signal and an input signal such that the phase and frequency of the output signal and the input signal tend to be the same. A PLL may also detect changes in a phase and frequency and adjust the phase and frequency to obtain an output signal having a fixed phase and frequency.
Analog PLLs are widely used in a variety of transceivers and devices. Recently, attempts have been made to reduce the size of a system-on-a-chip (SoC), and supply voltage has also been reduced. However, analog PLLs may be increasingly sensitive to external noise in such a case.
In order to achieve more accurate PLL control, a digitally controlled oscillator (DCO) has been proposed. Generally, a DCO is based on an inductor capacitor (LC) oscillator, to obtain an oscillating frequency represented by the following Equation 1 according to the inductance L and the capacitance C of the DCO:
                    f        =                              1                          2              ⁢              π              ⁢                              LC                                              .                                    [                  Equation          ⁢                                          ⁢          1                ]            
Accordingly, the DCO based on the LC oscillator is able to tune to an oscillating frequency by changing the capacitance.
However, the DCO for changing the capacitance has a resolution of the DCO, that is, a resolution of an output frequency of the DCO according to an input signal, limited by the capacitance.
Additionally, the DCO for changing the capacitance requires a plurality of capacitors in order to output various oscillating frequencies, but having a plurality of capacitors may be inefficient in costs and size. Furthermore, even where a plurality of capacitors is used, it may be impossible to reduce the intervals between neighboring frequencies below a predetermined range. Accordingly, it is difficult to implement an oscillator having a high resolution.